This invention relates to an endoprosthesis for femoral or tibial joint bone sections and adjacent femoral or tibial bone sections.
U.S. Pat. No. 4,578,081 discloses a bone prosthesis assembly for replacement of at least part of a natural joint of a patient wherein joint components can be connected to an elongation piece through a conical plug-in connection and the cones of the parts of the prosthesis define a self-locking connection. Since the prosthesis comprises a shank portion and a connection member adapted to be inserted in the bone by a set of elongation pieces of varying lengths, a relatively fine adaptation to varying lengths of the bone sections to be substituted can be made with a minimum stock expense. The self-locking cone connection provides a sufficient retaining force against the movement of the parts away from each other. Further, a considerable torque can be transferred thereby.
The above prosthesis can be varied in its length by using elongation pieces or joint components of different lengths. If an extension is necessary, a surgical operation must be carried out. Growing patients occasionally have to undergo multiple operations if the bone to be treated is to be adapted in its length to the length of the sound bone.
Therefore, it has been proposed to make a bone endoprosthesis continuously variable in its length to facilitate surgical operations. The endoprosthesis provided for this purpose consists of two telescopically co-operating rod portions. For example, one tubular rod portion is integrally formed with the joint portion and for example provided with an outer cone for the reception of a ball for a hip joint. The rod portion rotatably supports a nut co-operating with a threaded spindle of a second rod portion. A knee joint portion can be mounted, for example, on the outer end of the threaded spindle. The spindle nut is provided with an outer bevel-gear adapted to co-operate with the bevel-gear of a tool and is known in connection with drill chucks or the like.
The last-mentioned endoprosthesis has some deficiencies. A step is formed between both rod portions by which the patients may be affected: further, the soft tissue can be injured. The threaded spindle is directly located in soft tissue so that the latter may ingrow in the threads and disturb the adjustment of the threaded spindle. Conversely, adjustment of the threaded spindle may hurt the soft tissue. The adjustment drive for the threaded spindle is located between the ends of the endoprosthesis and thus is surrounded more or less by soft tissue. A post-operative adjustment of the length of the endoprosthesis thus requires a relatively extensive operation again.
In contrast to the above prior art, the present invention provides an endoprosthesis which is variable in length, can be simply adjusted post-operatively and does not injure soft tissue when adjusted and used.